In a switching power supply (for example, see JP 2017-139837 A.), second side output voltage is fed back to a primary side control IC via a photo coupler. The control IC adjusts output voltage of the switching power supply by changing duty of ON/OFF by a switching element in accordance with the voltage of a feedback terminal.
Herein, in an amplification device which includes the switching power supply and an amplifier to which voltage from the switching power supply is supplied, to improve audio quality and so on by suppressing feedback amount to the control IC, gain is set to small. As a result, response characteristics deteriorate, and performance of the switching power supply deteriorates.